Multiple-screw extruder

ABSTRACT

A multiple-screw extruder which has a central feed screw meshing with two or more outer screws uniformly spaced around the circumference of the central screw, its outer screws having a mean thread diameter equal to a mean thread diameter of the central screw divided by the number of outer screws.

I United States Patent [151 3,640,669

Hanslik 5] Feb. 8, 1972 [54] MULTIPLE-SCREW EXTRUDER 2,581,451 l/1952Sennet ..18/l2 SP UX 2,802,238 8/1957 Co1ombo.. ..l8/l2 SP [72] Ausma3,043,480 7/1962 Wittrock ....1s/12 SP ux [73] Assignee: DorplastexA.G., Zug, Switzerland 3,248,469 4/1966 Kosinsky et 611.. 18/12 SM X3,310,837 3/1967 Wittrock ....18/l2 SP [22] 1969 3,325,864 6/1967Kohyama etal ..l8/l2 SP 211 App]. No.2 874,236

Primary Examiner]. Spencer Overholser Assistant Examiner-Michael 0.Sutton [30] Fore'gn Applicant) Pnomy Dam Attorney-Keenig, Sanniger,Powers & Leavitt Nov. 18, 1968 Austria ..A 11202/68 ABSTRACT ((51 Amultiple screw extruder which has a central feed screw meshing with twoor more Outer Screws uniformly spaced [58] Field ofSearch ..l8/l2 SP, 12SM, 12 SE, 30 SM around the circumference of the central Screw its Omarscrews having a mean thread diameter equal to a mean thread [56]References cued diameter of the central screw divided by the number ofouter UNITED STATES PATENTS screws- 2,550,226 4/1951 Colombo ..l8/ 12 10Claims ,3Drawing Figures mcmenm 6 I912 3.640.669

, FIG./ l2 13 F162 4 6 "I' l F7 lz/i I! 5 INVENTOR ilhelm 646M611 BYMAM,

Ms ATTORNEY.

MULTIPLE-SCREW EXTRUDER BACKGROUND OF THE INVENTION This inventionrelates to a multiple-screw extruder for thermoplastic materials or thelike.

Extruders having two meshing feed screws driven in opposite directionsare known. The dimensions of the threads of the meshing feed screws arelimited by the condition that the cross section of the thread must notbecome excessive as this would interfere with the even heating of thematerial being extruded and with its homogenization. The feed rate ofthese twin screw extruders is limited, when used with heat-sensitiveplastics, for example, to a maximum flow of about 300 kg. per hour inthe processing of pure polyvinyl chloride. If heavy sections are to beproduced having a considerable weight per unit length, this performanceis no longer adequate for efficient working. If the section to beproduced is to have a weight of 80 kg. per meter, a rate of only a fewcentimeters per minute is obtained, so that the installation with itscomparatively high initial costs no longer operates economically. Thereis also the difficulty that extruders for large outlet cross sectionstake up considerable space so that the plastic material is likely todwell for a fairly long time in the extruder, on account of thecomparatively low rate of feed. There is accordingly a danger that thematerial may decompose in the extruder. It has therefore already beenproposed to use several such twin screw extruders in parallel. However,quite apart from the costs of an installation of this kind, thecombining of the extruded materials presents difficulties and there isalso a danger that the extruded material is no longer of homogeneousstructure. These arrangements with several twin feed screw extruders canaccordingly be used only for processing insensitive materials, such aspolyvinyl chloride mixed with a filler.

It has also been suggested to provide an extruder with more than twofeed screws meshing with each other in the sense that several feedscrews mesh with a common central screw. However, these knownarrangements have the disadvantage that the rate of feed of the centralscrew differs from that of the outer screws so that these arrangementshave not been successful.

SUMMARY OF THE INVENTION The present invention involves a multiple-screwextruder having a central feed screw meshing with two or more outerscrews uniformly spaced around the circumference of the central screwwherein the outer screws have a mean thread diameter equal to a meanthread diameter of the central screw divided by the number of outerscrews. The mean thread diameter of the outer screws is defined as theirpitch diameter, and that of the central screw is defined as a diameterbetween its pitch diameter less one-third of the depth of the thread andits pitch diameter plus one-third of the depth of the thread andpreferably between its pitch diameter less one third of the depth ofthread and its pitch diameter. With this arrangement approximately equalconditions are created for the plastic material conveyed by the centralfeed screw and that conveyed by the outer screws because the threads ofthe outer screws, from end to end of the enclosed extruder space, are ofapproximately the same length as those of the central feed screw. If,for instance, three outer screws are arranged about one central screwand the mean diameter of the outer screws is about one-third of that ofthe central screw, the points of engagement of the outer screws with thecentral screw divide the central feed screws circumference into threeequal parts or segments of a length of arc corresponding approximatelyto the circumference of the outer screws. However, since the threads inthe outer screws have a greater curvature, the volume of the grooving ofthese threads is rather less than that of the central feed screw withits smaller curvature, in the area between the points of engagement oftwo adjacent outer screws. It is therefore preferable if the meandiameter of the outer screws is taken as their pitch diameter, and thatof the central feed screw as one between its pitch diameter lessonethird of the depth of the thread and its pitch diameter, so that thevolumes of the plastic material enclosed in the groove of one thread ofan outer screw and that in the groove of one thread of the central feedscrew correspond more closely to each other, in view of the somewhatlarger size of the central feed screw.

The diameters of the outer screws can be made approximately equal, as inthe case of a twin feed screw extruder, so that the same conveyingconditions can be created as with a twin feed screw extruder. If, forinstance, five outer screws are arranged round a central feed screw, therate of feed of a multiple-screw extruder of this kind is five timesthat of a twin feed screw extruder, where the feed screws are of thesame size as the outer screws of the extruder according to theinvention. This yields good driving conditions because the large centralscrew tends to float between the outer screws, and car riessubstantially no radial load, while the comparatively small outer screwscarry radial loads, but not more than in the case of a twin feed screwextruder of only a fraction of the performance.

For many materials it is necessary to arrange the material compositionto suit the machine. Since in the case of the mul tiple feed screwextruder according to the invention, approximately the same conveyingconditions are obtained in the threads of the outer screw and in thethread parts of the central screw, as in the case of a twin feed screwextruder with a single pair of feed screws, these feed screwscorresponding to the outer feed screws of the multiple model, each newcomposition can be tried out in a twin feed screw extruder or perhaps ina multiscrew extruder according to the invention with only two outerscrews, and then used without change in a multiscrew extruder accordingto the invention, with a large number of outer screws. In this way thecomposition can be tested on a small scale so that less material islost.

The more effectively the conveying conditions in the threads of theouter screws are adapted to those in the thread parts of the centralscrew, the more accurately the material fed by the outer screws agreeswith that fed by the central screw, and the more homogeneous is thematerial leaving the extruder. in order to ensure even greater agreementbetween the conveying conditions in the outer screws and in the centralscrew, the number of threads of the central screw may be made equal tothe number of threads of the outer screws multiplied by the number ofouter screws, and also, the central screw and the outer screws can bemade with approximately the same helix angle. Moreover, according to afurther feature of the invention it is convenient to make the depth ofthread and width of thread of the central screw equal to those of theouter screws, the threads in the central screw and the outer screwshaving preferably the same section.

In order to ensure further homogenization of the material leaving theextruder, the invention further proposes extending the central screw onthe outlet side of the machine, beyond the length of the outer screws,to form a mixing head, the pitch of the central screw at the part whereit projects beyond the outer screws being at least twice the pitch ofthe central screw at the part of this screw where it cooperates In theouter screws so that the rate of feed of this screw at the mixing headis increased. If the pitch of the feed screw in the mixing head, withabout the same diameter as that of the central screw, is twice the pitchof the threads in the central screw, the rate of feed of the mixing headof this screw should correspond to the total combined rates of feed ofthe central screw and the outer screws, if the mixing head screw is anonslip one. But since this is not the case and the material in themixing end slips, it is convenient to make the pitch of the screwthreads in the mixing head more than twice that of the screw threads ofthe central screw.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described byway of example, with reference to the accompanying drawings, in which:

FIG. 1 is a longitudinal section through an extruder along the line 1-1of FIG. 2,

FIG. 2 is a cross section along the line 1II1 of FIG. 1, and

FIG. 3 shows an axial section through a screw thread, on a larger scale.

DESCRIPTION OF THE PREFERRED EMBODIMENT The reference 1 denotes a screwbarrel and 2 the extruder outlet aperture. At the end of screw barrel 1opposite the outlet aperture 2, an admission duct for the material to beextruded is provided in the usual manner. The material is athermoplastic one, for instance, polyvinyl chloride.

A central feed screw 3 with a relatively large diameter is accommodatedin the barrel and feed screws 4 of smaller diameter, denoted here by theterm outer screws, mesh with it. The outer screws 4 are uniformly spacedaround the circumference of the central screw 3. In the embodiment shownthere are three outer screws 4, so that they are spaced at 120". If, forinstance, five outer screws are provided, the spacing would be 72. Thescrews extend longitudinally in the barrel. The mean diameter of theouter screws 4 corresponds to the mean diameter of the central screw 3divided by the number N of outer screws provided. In the embodimentshown with three outer screws 4, the mean diameter of these outer screwsis accordingly one-third of the diameter of the central screw 3. Thenumber of threads of the central screw 3 is equal to the number ofthreads of the outer screws 4, multiplied by the number N of outerscrews. Since the threads of the outer screws 4 are single-start ones,in the present case, the central screw 3 would have three starts (threethreads). The central screw 3 and the outer screws 4 are positivelydriven at fixed relative speeds so that the outer screws 4 rotate atthree times the speed of the central screw 3. The helix angle of thecentral screw 3 is approximately equal to that of the outer screws 4.

The circumference of central screw 3 is about three times that of eachouter screw 4. The starts or individual threads of the central screw aretherefore divided by the outer screws 4 into parts or segments a, eachbeing as long as the free threads of the outer screws 4, indicated bythe symbol b. The threads end each in points 5 and 6, so that thelengths a and b are measured each from the point 6 to the point 5. Thismeans that the points of engagement at the points 5 and 6 separateapproximately equal volumes of the grooving defined by the threads inthe central feed screw 3 and in the outer screws 4, so thatapproximately equal conveying conditions are created.

In FIG. 3, a thread 7 of a screw is shown in section, on a larger scale,and the grooving defined by the thread is indicated at 7a. Thisillustration is applicable both to the outer screws and to the centralscrew. Reference 9 is the cylindrical wall of the barrel 1 surroundingthe worm thread 7. The core diameter of the screw is indicated by c, thepitch diameter by d, and the outside diameter by e. The feeding rate ofthe screw depends largely on the ratio of the moving parts of thecircumference of the screw thread 7 to its nonmoving parts. The base 8of the grooving defined by the screw thread and the flanks 11 are movingso as to convey the medium. The cylindrical wall 9 of the barrel isstationary and acts as a brake. The ratio of the moving surface 8 and 11to the stationary surface 9 is governed by the diameter of the screw andthis diameter is in the case of the central screw 3 with its largediameter greater than in the outer screws 4 of similar diameter. Themean diameters to be used of the central screw and the outer screws,taken in the embodiment shown in the drawing in the ratio 3/ 1, areaccordingly not accurately known. Approximately the right result isobtained if in the case of the outer screws the mean diameter is takenas the pitch diameter d, and in the case of the central screw a diameteris selected from the range limited by the pitch diameter d lessone-third of the depth of thread f, on the one hand, and the pitchdiameter d plus a third of the depth of thread f, on the other. In thisrange, amounting to two-thirds of the depth of thread f, favorableconditions are found. However, it is convenient to take, as the meandiameter to be used in a ratio of 1/3, the pitch diameter d, for theouter screws, and for the central screw a diameter lying between thepitch diameter d less one-third of the depth of thread f, and the pitchdiameter d. This means that the length of the thread, measured on thepitch diameter of the central screw 3, is slightly shorter than in theouter screws 4. This takes into account the fact, already mentionedabove, that in the central screw 3 with its larger diameter the ratio ofthe radially moving surfaces 8 and 11 to the stationary surface 9 isgreater than in the outer screw 4 with its smaller core diameter so thatin the central screw friction and adhesion of the material bring aboutcomparatively more effective feed in the radial direction, and that thefeed of the outer screw is less than that of the larger central screw 3because of the greater curvature of the pitch circle of the smaller sidescrews, so that the volume of a thread of an outer screw is less thanthat of a central screw, for a pitch arc of the same length.

The more the screws are arranged to suit each other, the more theconveying conditions in the grooving 7a of the threads 7 of the centralscrew 3 are assimilated to those of the grooving of the threads 7 of theouter screws 4. In order to achieve further homogenization of theextruded material, the central screw 3 is extended beyond the outerscrews 4 to form a mixing head 12. This mixing head 12 rotates in acylindrical extension 13 of the screw barrel 1 and has screw threads 14of a lead more than twice that of the screw threads 7.

The drive of the central screw is effected in the usual manner by agearwheel 15 on a shaft 16 of the central screw 3. This shaft 16 alsohas a pinion l7 engaging with pinions 18 on shafts 19 of the outerscrews 4. In this way the outer screws 4 are driven at the correct ratioof the central screw 3. The thrust of the central screw is absorbed by athrust bearing 20 and the thrusts of the outer screws 4 by thrustbearings 21.

What I claim is:

l. A multiple-screw extruder comprising a barrel having an outlet at oneend for extrusion therethrough of material admitted to the barrel, acentral feed screw extending longitudinally in the barrel, a number N ofouter feed screws extending longitudinally in the barrel spaced atsubstantially equal intervals around the central screw and each in meshwith the central screw, said screws being adapted upon rotation thereofin the barrel to feed material forward in the grooving defined by thethreads of the screws toward and out through said outlet, and means fordriving the screws for feeding material forward in said grooving, eachof the outer screws having a mean thread diameter equal to a mean threaddiameter of the central screw divided by N, the mean thread diameter ofeach outer screw being taken as its pitch diameter, and the mean threaddiameter of the central screw being a diameter in the range between thepitch diameter of the central screw less l/N times its thread depth andthe pitch diameter of the central screw plus l/N times its thread depth.

2. A multiple-screw extruder according to claim 1 in which the number ofthreads of the central screw is equal to the number of threads of theouter screws multiplied by the number N of outer screws.

3. A multiple-screw extruder according to claim 2, in which the centralscrew and the outer screws have approximately the same helix angle.

4. A multiple-screw extruder according to claim 3 in which the depth ofthread and width of thread of the central screw are equal to those ofthe outer screws.

5. A multiple-screw extruder according to claim 4 in which the threadsof the central screw and those of the outer screws have the same crosssection.

6. A multiple-screw extruder according to claim 5 in which the centralscrew is extended at the outlet end beyond the outer screws, to form amixing head, having a helix angle at least twice that of the centralscrew where it cooperates with the outer screws.

7. A multiple-screw extruder comprising a barrel having an outlet at oneend for extrusion therethrough of material admitted to the barrel, acentral feed screw extending longitudinally in the barrel, a pluralityof outer feed screws extending longitudinally in the barrel spaced atsubstantially equal intervals around the central screw and each in meshwith the central screw, said screws being adapted upon rotation thereofin the barrel to feed material forward in the grooving defined by thethreads of the screws toward and out through said outlet, and means forpositively driving all the screws for feeding material forward in saidgrooving, the threads of the central and outer screws being so relatedand the driving means being adapted to drive the outer screws at suchspeed relative to the central screw that the outer screws feed thematerial forward at substantially the same rate as the central screw,wherein the central screw has a plurality of threads equal in number tothe number of threads on each outer screw multiplied by the number ofouter screws, wherein the volume of the grooving of each outer screwover the length thereof is substantially equal to the volume of thegrooving of the central screw in a segment of the central screw betweentwo adjacent outer screws over the same length, wherein the length ofeach thread of the outer screws is approximately equal to the length ofeach thread of the central screw and wherein the threads of all thescrews are of substantially the same cross section, wherein the helixangles of the central screw and each outer screw are approximatelyequal, wherein the number of outer screws equals N and wherein eachouter screw has a mean thread diameter substantially equal to UN timesthe means thread diameter of the central screw, the mean thread diameterof each outer screw being taken as its pitch diameter, and the meanthread diameter of the central screw being a diameter in the rangebetween the pitch diameter of the central screw less l/N times itsthread depth and the pitch diameter of the central screw plus l/N timesits thread depth.

8. A multiple-screw extruder as set forth in claim 7 wherein there arethree outer screws and wherein each outer screw has a mean threaddiameter substantially equal to one-third the mean thread diameter ofthe central screw, the mean thread diameter of each outer screw beingtaken as its pitch diameter, and the mean thread diameter of the centralscrew being a diameter in the range between the pitch diameter of thecentral screw less one-third of its thread depth and the pitch diameterof the central screw plus one-third its thread depth.

9. A multiple-screw extruder as set forth in claim 7 wherein the numberof outer screws equals N and wherein each outer screw has a mean threaddiameter substantially equal to UN times the mean thread diameter of thecentral screw, the mean thread diameter of each outer screw being takenas its pitch diameter, and the mean thread diameter of the central screwbeing a diameter in the range between the pitch diameter of the centralscrew less l/N times its thread depth and the pitch diameter of thecentral screw.

10. A multiple-screw extruder as set forth in claim 7 wherein there arethree outer screws and wherein each outer screw has a mean threaddiameter substantially equal to onethird the mean thread diameter of thecentral screw. the mean thread diameter of each outer screw being takenas its pitch diameter, and the mean thread diameter of the central screwbeing a diameter in the range between the pitch diameter of the centralscrew less one-third of its thread depth and the pitch diameter of thecentral screw.

1. A multiple-screw extruder comprising a barrel having an outlet at oneend for extrusion therethrough of material admitted to the barrel, acentral feed screw extending longitudinally in the barrel, a number N ofouter feed screws extending longiTudinally in the barrel spaced atsubstantially equal intervals around the central screw and each in meshwith the central screw, said screws being adapted upon rotation thereofin the barrel to feed material forward in the grooving defined by thethreads of the screws toward and out through said outlet, and means fordriving the screws for feeding material forward in said grooving, eachof the outer screws having a mean thread diameter equal to a mean threaddiameter of the central screw divided by N, the mean thread diameter ofeach outer screw being taken as its pitch diameter, and the mean threaddiameter of the central screw being a diameter in the range between thepitch diameter of the central screw less 1/N times its thread depth andthe pitch diameter of the central screw plus 1/N times its thread depth.2. A multiple-screw extruder according to claim 1 in which the number ofthreads of the central screw is equal to the number of threads of theouter screws multiplied by the number N of outer screws.
 3. Amultiple-screw extruder according to claim 2, in which the central screwand the outer screws have approximately the same helix angle.
 4. Amultiple-screw extruder according to claim 3 in which the depth ofthread and width of thread of the central screw are equal to those ofthe outer screws.
 5. A multiple-screw extruder according to claim 4 inwhich the threads of the central screw and those of the outer screwshave the same cross section.
 6. A multiple-screw extruder according toclaim 5 in which the central screw is extended at the outlet end beyondthe outer screws, to form a mixing head, having a helix angle at leasttwice that of the central screw where it cooperates with the outerscrews.
 7. A multiple-screw extruder comprising a barrel having anoutlet at one end for extrusion therethrough of material admitted to thebarrel, a central feed screw extending longitudinally in the barrel, aplurality of outer feed screws extending longitudinally in the barrelspaced at substantially equal intervals around the central screw andeach in mesh with the central screw, said screws being adapted uponrotation thereof in the barrel to feed material forward in the groovingdefined by the threads of the screws toward and out through said outlet,and means for positively driving all the screws for feeding materialforward in said grooving, the threads of the central and outer screwsbeing so related and the driving means being adapted to drive the outerscrews at such speed relative to the central screw that the outer screwsfeed the material forward at substantially the same rate as the centralscrew, wherein the central screw has a plurality of threads equal innumber to the number of threads on each outer screw multiplied by thenumber of outer screws, wherein the volume of the grooving of each outerscrew over the length thereof is substantially equal to the volume ofthe grooving of the central screw in a segment of the central screwbetween two adjacent outer screws over the same length, wherein thelength of each thread of the outer screws is approximately equal to thelength of each thread of the central screw and wherein the threads ofall the screws are of substantially the same cross section, wherein thehelix angles of the central screw and each outer screw are approximatelyequal, wherein the number of outer screws equals N and wherein eachouter screw has a mean thread diameter substantially equal to 1/N timesthe means thread diameter of the central screw, the mean thread diameterof each outer screw being taken as its pitch diameter, and the meanthread diameter of the central screw being a diameter in the rangebetween the pitch diameter of the central screw less 1/N times itsthread depth and the pitch diameter of the central screw plus 1/N timesits thread depth.
 8. A multiple-screw extruder as set forth in claim 7wherein there are three outer screws and wherein each outeR screw has amean thread diameter substantially equal to one-third the mean threaddiameter of the central screw, the mean thread diameter of each outerscrew being taken as its pitch diameter, and the mean thread diameter ofthe central screw being a diameter in the range between the pitchdiameter of the central screw less one-third of its thread depth and thepitch diameter of the central screw plus one-third its thread depth. 9.A multiple-screw extruder as set forth in claim 7 wherein the number ofouter screws equals N and wherein each outer screw has a mean threaddiameter substantially equal to 1/N times the mean thread diameter ofthe central screw, the mean thread diameter of each outer screw beingtaken as its pitch diameter, and the mean thread diameter of the centralscrew being a diameter in the range between the pitch diameter of thecentral screw less 1/N times its thread depth and the pitch diameter ofthe central screw.
 10. A multiple-screw extruder as set forth in claim 7wherein there are three outer screws and wherein each outer screw has amean thread diameter substantially equal to one-third the mean threaddiameter of the central screw, the mean thread diameter of each outerscrew being taken as its pitch diameter, and the mean thread diameter ofthe central screw being a diameter in the range between the pitchdiameter of the central screw less one-third of its thread depth and thepitch diameter of the central screw.